Insulation Calculator
Calculate insulation coverage for walls, attics, and floors by area. Enter the space dimensions to get the square footage to insulate and how many batts or bags that takes. Helps you hit a target R-value without over-buying.
Insulation Diagram
How the Formula Works
Calculate the insulation area
Area = Length × Width For walls, multiply the wall run (total linear feet) by the wall height. For attics and floors, multiply room length by room width. Deducting window and door openings is optional but adds accuracy — each standard door is roughly 20 ft² and each window 6–15 ft².
Apply waste factor
Adjusted Area = Area × (1 + Waste %) A 10% waste allowance is standard for typical walls. Use 15% for attics with obstructions like cross-bracing, plumbing vents, and HVAC ducts. Offcuts around electrical outlets, pipes, and corner framing are unavoidable — the waste factor ensures you do not run short mid-project.
Find coverage per roll or batt
See product label — common: R-13 batt ≈ 40–48 ft² | R-19 batt ≈ 48–65 ft² Always read the coverage from the product label rather than estimating. Coverage varies by brand, thickness, and width — a 15″ wide batt covers a different area than a 23″ wide batt even at the same R-value. Rolls cover more per unit than pre-cut batts and suit large unobstructed areas like attic floors.
Calculate rolls or batts required
Rolls = ⌈Adjusted Area ÷ Coverage per Roll⌉ Divide the adjusted area by the coverage per roll and always round up to the nearest whole number — you cannot buy a fraction of a roll or batt. Pre-cut batts are sized to fit standard stud bays (16″ or 24″ OC); rolls are cut to length on-site. Both are calculated the same way by area.
Understand R-value
Higher R = more thermal resistance R-13 is standard for 2×4 walls (3.5″ cavity); R-19 or R-21 for 2×6 walls (5.5″ cavity). Attics in cold climates typically require R-38 to R-49. R-value is additive when layering — two layers of R-19 batt yield R-38. Check your local building code for minimum R-value requirements by climate zone.
Frequently Asked Questions
Measure the length and width of the area to insulate and multiply them to get the square footage. Add a 10% waste factor for cuts around outlets, windows, and doors. Divide the adjusted area by the coverage per roll or batt (shown on the product label) and round up to the nearest whole number. This calculator does all of that automatically.
R-13 is the standard for 2×4 exterior walls; R-19 or R-21 for 2×6 walls. The required R-value depends on your climate zone — colder climates require higher values. Check your local building code (IRC Table N1102.1.2 in the US) for the minimum R-value for your zone. Higher R-values reduce heating and cooling costs but cost more upfront.
Batts are pre-cut to standard lengths (typically 93″ or 105″) sized to fit between studs or joists at standard spacing. Rolls are the same material in continuous lengths that you cut to size on-site. Batts suit framed walls and ceilings with consistent stud spacing; rolls are more economical for attic floors with fewer obstructions. Both are calculated the same way by area coverage.
At R-38 using batts with 40 ft² coverage per roll: 1,000 ft² × 1.10 waste factor = 1,100 ft² adjusted ÷ 40 ft² per roll = 28 rolls (rounded up). For R-38 you may need two layers of R-19 — calculate each layer separately. Actual coverage varies by product; always check the label.
Yes. A 10% waste factor is standard for typical walls and floors. Use 15% for attics with cross-bracing, plumbing stacks, and HVAC penetrations. Cuts around electrical boxes, corner framing, and window/door rough openings generate offcuts that cannot be reused. Skipping the waste factor often leaves you one or two rolls short at the end of the job.
R-13 batt insulation in 15″ width typically covers 40–48 ft² per bag (usually 10 batts at 93″ length). In 23″ width it covers 65–75 ft². Continuous rolls cover more — an R-13 roll can cover 87–115 ft² depending on width. Coverage varies by brand and product line, so always read the label rather than relying on averages.
A 2×4 wall has a 3.5″ stud cavity — R-13 or R-15 batts fit perfectly. A 2×6 wall has a 5.5″ cavity — use R-19 or R-21 batts. Using an R-13 batt in a 2×6 cavity leaves an air gap and reduces effective performance. You can add rigid foam board to the exterior to increase total wall R-value without changing framing.
This calculator is designed for batt and roll insulation priced and measured by area coverage. Spray foam is sold by the board-foot (1 ft² at 1″ thickness) or by two-component kit coverage in square feet at a given depth. Use a spray foam kit's stated coverage in ft² at your target depth as the "coverage per roll" input for a rough estimate, but for spray foam projects a dedicated board-foot calculator is more accurate.